Ramanujacharya didn't write a commentary on the Brahma Vaivarta Purana. In fact Ramanujacharya didn't write any Purana commentaries at all. As described in this excerpt from Vedanta Desikan's Rahasyatraya Sara, he only composed nine works: the Sri Bhashya, the Vedanta Dipa, and the Vedanta Sara, three commentaries on the Brahma Sutras; the Bhagavad Gita Bhashya, a commentary on the Bhagavad Gita; the Vedartha Sangraha, an independent exposition of Visistadvaita; the Sharanagati Gadyam, the Sri Ranga Gadyam, and the Vaikuntha Gadyam, three prose hymns in praise of Vishnu; and the Nityam, a work about the daily activities of Sri Vaishnavas. (All these works are available in English except the Nityam.). He didn't compose commentaries on any Puranas including the Vishnu Purana.
Other Sri Vaishnava Acharyas did write Purana commentaries, but only on the Vishnu Purana and Srimad Bhagavatam as far as I know. But Ramanujacharya does quote from the Brahma Vaivarta Purana, in this section of the Sri Bhashya:
Scripture declares, 'no work the fruits of which have not been completely enjoyed perishes even in millions of aeons.'
As this forum post says, this verse is from the Prakriti Kanda of the Brahma Vaivarta Purana.
But Ramanujacharya does not quote or discuss the Brahmvaivarta Purana declaring Krishna to be supreme and Vishnu to be an incarnation of Krishna. Whereas he and numerous other Sri Vaishnava Acharyas discuss the Shaiva Purana passages declaring Shiva to be supreme.